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FM radio

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where it says, "The ability to tune 88.9 MHz and 89.1 MHz is testimony of its selectivity." does it mean it can reach those, or does it mean that its range is only 88.9 MHz to 89.1 MHz, coz im after osmething that can get 95.8-106.2 at least, if it is a wide range this is amazing! the best one ive seen yet thanks alot!
 

stevez

Active Member
I took a look at the site -that's one nice, simple project!!

Selectivity generally describes the ability of a receiver to separate other signals in the band. Usually with a simpler receiver the selectivity is such that two radio stations that are close in frequency will both be heard unless one is much weaker than the other. I could be wrong but this receiver is likely to be capable of receiving across the entire FM band. If it can allow listening to 88.9 while a station exists on 89.1 that's not bad. Keep in mind that it's still a simple receiver. Staying tuned to the frequency of interest might be one problem. Overload in a major metro area might be another.
 

vaineo

New Member
I'm almost positive is gets the entire FM radio band thats just showing you the step size it tunes.
 

DigiTan

New Member
Yeah, the text says at the bottom that you can adjust the tuning using the 15-120pf capacitor--or by compressing the coil L1 with your hand.

I'm curious to know how digital tuners work. Is it possible to apply a digital tuning device to this design as well?
 

Nigel Goodwin

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DigiTan said:
Yeah, the text says at the bottom that you can adjust the tuning using the 15-120pf capacitor--or by compressing the coil L1 with your hand.

I'm curious to know how digital tuners work. Is it possible to apply a digital tuning device to this design as well?
Digital tuning uses a PLL (Phase Locked Loop), basically the oscillator has to be a VCO (Voltage Controlled Oscillator). You divide the oscillator frequency down using a programmable divider, and compare it to a reference frequency - it makes life easier if the reference frequency is the channel spacing, but this isn't essential. The frequency comparator produces an error signal, this is used to change the voltage to the VCO, altering it's frequency. When the VCO reaches the corrent frequency there's no error produced, and the PLL is locked on frequency - if it drifts at all the PLL will correct it.

The crude super-regen circuit isn't suitable for digital tuning, you need a superhet design (which has a local oscillator) to do so.
 

Nigel Goodwin

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-=GST=- Nemisis (cs/cz) said:
i just about understood that :? could you say what to do in a simpler way, or where i can get a digital superhet design like you said and how to install it.
A quick google found this on-line book, which you may find useful
http://www.mikroelektronika.co.yu/english/product/books/rrbook/rrbook.htm

Building a PLL tuned FM superhet isn't a trivial task, and it's likely to cost considerably more than buying one - you also have the problems of layout (which is fairly critical at VHF) and of alignment.
 

stevez

Active Member
A few of the electronics suppliers (might be Ocean State Electronics or Jameco) sell a really nice AM/FM or FM radio kit. Seems to me that Ramsey also sells a kit. Each of these are fairly substantial designs. You might find that modifying the kits would relieve you of considerable burden. As already suggested, VHF circuits may not look complex in terms of a schematic however construction methods and details are critical.
 
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